1. Field of the Invention
The present invention relates to an acceleration device for charged particles. It also relates to an accelerator system incorporating such a device.
2. Summary of the Prior Art
It is known to generate synchrotron radiation using a ring type accelerator as the synchrotron radiation generator. In a synchrotron accelerator or in a storage ring, a beam of charged particles is accelerated to a storage energy. In order to do that, particles at low energy are obtained, and injected into the ring for acceleration to high energy. When synchrotron radiation is needed for industrial purposes, it becomes important that the synchrotron radiation generator is relatively compact. Generally, an industrial synchrotron radiation generator has a linear accelerator which creates a beam of charged particles and accelerates it to a low energy level, a synchrotron which raises the low energy charged particle beam to a higher energy level, and an accumulation ring which accelerates the beam even further and accumulates the beam of charged particles.
As stated above, it is desirable that an industrial synchrotron radiation generator occupies a small area. This enables the generator to be installed in e.g. a semiconductor fabrication factory. A high brightness (i.e. large current) is also necessary to reduce the irradiation time. To meet the requirement of a small area it is, of course, necessary to make each unit element smaller. However, if by using only an accumulation ring, a charged particle beam can be synchrotron accelerated from a low energy level to a final energy level in a stable way, the synchrotron stage can be omitted and the size of the system reduced significantly.
A charged particle beam is accelerated with energy supplied from a high frequency power source through a high frequency (radio frequency) acceleration cavity. To achieve stable synchrotron acceleration of a charged particle beam with a high frequency acceleration cavity, synchrotron phase stability (hereinafter referred simply to as phase stability, which will be explained in more detail later) must be achieved. When a charged particle passes through a high frequency acceleration cavity, an electric field is created by this current, and with this electric field, a voltage is generated in opposite phase to the acceleration voltage which is generated from the high frequency power source (hereinafter this voltage in opposite phase is referred to as the voltage induced by the beam). As a result, the charged particles lose a part of the energy supplied and it becomes difficult to ensure the stability of the beam around the looped path. Thus, the charged particles cannot maintain a satisfactory phase stability. Such an effect becomes greater as the number of charged particles in the beam increases, i.e. as the beam current increases. Hereinafter, the gap between the oscillation frequency of the high frequency power source and the resonance frequency of the high frequency acceleration cavity will be referred to as the de-tune value, and the creation of such gap as detuning.
One method of synchrotron acceleration of charged particles is discussed in the study "Characteristics of a high frequency acceleration cavity" (INS-TH-96. Institute of Nuclear Study, Tokyo University, Feb. 18, 1975). This conventional technology adopts the method of maintaining a constant acceleration voltage to the charged particles by controlling the high frequency power only, which is the source of the power supply to the high frequency acceleration cavity.
A high frequency acceleration cavity is discussed in the IEEE Partial Accelerator Conference (1987) pp. 1901 to 1903. To change the resonance frequency, the high frequency acceleration cavity must be transmitted onto the magnetic body which consists of a tuner. The aforementioned conventional technology uses a method of capturing the high frequency magnetic field in a cavity then transmitting it by using a coaxial transmission line.
In the high frequency acceleration cavity discussed above, the capturing of the high frequency magnetic field was via a coaxial cable, and this method permitted only a small change in the detuning. In low current applications, this is not a problem, but it becomes so at higher current where the amount of detuning is greater.